1923 ] Esterly: Marine Copepoda at La Jolla 431 



by night than by day. The evidence on this point is overwhelming 

 for the two species of Acartia. The mixed groups also are present in 

 larger numbers at night, though the differences here are not so signifi- 

 cant as for Acartia. Since all the hauls we're made at the surface, it is 

 not possible to state where most of the animals are during the day; 

 but there cannot be any doubt that the appearance of the larger num- 

 bers at night is due to an actual increase in the surface population, 

 and not to inability of the animals to avoid the net in the dark. 



The diurnal migration of plankton animals is such a well-known 

 phenomenon that much experimental work has been done in making 

 attempts to ascertain the causes of their movements. Acartia clausi 

 and A. tonsa have been worked with in the laboratory (Esterly," 1917 ; 

 1919, pp. 16-22), and it has seemed from the experiments made there 

 that a possible factor in the migrations is a physiolagical rhythm. 

 Responses to external stimuli, at any rate, do not afford an adequate 

 explanation. So far as the study of the pier collections has gone, the 

 cause of the greater abundance at the surface at night is not indicated, 

 although the fact of the diurnal change is firmly established. It remains 

 to be seen whether more elaborate correlations will give a clue to the 

 reason for the daily rhythm in the sea. 



Nevertheless, as has been so often urged in publications from the 

 Scripps Institution, the natural history of the plankton cannot be 

 known until abundant records of both field and experimental obser- 

 vations are available. No amount of experimental work could show 

 that tonsa is most abundant in July and August and clausi in March. 

 And field observations would never bring to light the physiological 

 rhythm that obtains for both species. It is now possible, for the first 

 time, to set about an investigation of the habits of these two plankton 

 copepods in the light of accurate field data as well as of experimental 

 data. 



It may be of interest to call attention to a suggestion that came 

 from the laboratory observations. It was thought that the behavior 

 of the two species of Acartia gave reason for believing that the 

 numbers of animals at the surface would be greater during the colder 

 months (Esterly, 1919, p. 22). But our field data show that only 

 clausi is most abundant when the water is coldest (March is one of 

 the three coldest months), while tonsa is most abundant in July and 

 August when the water is warmest. This instance shows how neces- 

 sary it is to have field data to check experimental observations. 



In conclusion it should be again pointed out that the purpose in 

 mind in taking this series of collections was not to determine the 



